UInt64
From Xojo Documentation
Data Type
Used to store unsigned 64-bit integer values. The default value is 0. Generally you will use the Integer data type (equivalent to Int32 on 32-bit apps or Int64 on 64-bit apps) or UInteger (equivalent to UInt32 on 32-bit apps or UInt64 on 64-bit apps). This size-specific integer data type is available for use with external OS APIs.
Notes
UInt64 values can range from 0 to 2^64-1 (18,446,744,073,709,551,615) and use 8 bytes.